Plants heterosis

The phenomenon of heterosis was first described by Darwin as having a positive effect on plant traits, as displayed by cross-fertilized over self-fertilized progenies in the vegetable kingdom. Shull and East's rediscovery of heterosis in 1906 paved the way for the introduction of the first maize hybrid varieties in the US market in early 1930. Since then the occurrence of hybrid vigor has been proven for many plant species (allogamous and autogamous) and for several characteristics (biomass and resistance to biotic and abiotic stress). Genetic control studies on hybrid vigor support the different hypotheses that have been suggested and debated (dominance, over-dominance, epistasis) and, in general, it is accepted that the three mechanisms are not mutually exclusive. The advances in dissecting the molecular mechanisms underlying heterosis seem to indicate that new patterns of whole-genome coordinated activities differently modulated in the various sets of genes (e.g. syntenic/non-syntenic, dosage-dependent genes), plant tissues, developmental stages, and in timing (e.g., altered circadian-mediated gene expression) are involved in the higher performances of heterotic hybrids vs the parental lines. These new patterns of genome activity in hybrids can be the result of multiple molecular mechanisms documented in several recent studies: gene expression variation (differential/non-differential variation relative to the parental lines; additive/non-additive variation; cis- and trans-acting regulatory variation of parental alleles), gene complementation (from presence/absence variation and active/non-active variation between parents), epigenetic regulation (e.g., DNA methylation). The effort to tackle heterosis from the genetic, epigenetic, and molecular point of view, seems to have led to a more and more complex depiction of heterosis itself, hampering a generalized scientific theory to thoroughly cover the phenomenon. On the other hand, plant breeders keep on working successfully with breeding schemes in many crops and vegetables estimating parental genotype 'combining abilities' to catch the best 'well combining' parental lines and consequently the most heterotic hybrids. It is then clear that the genetic, epigenetic, and molecular dissection of the 'well combining' genotypes is also of great interest in the view of establishing a consistent heterotic pattern (i.e., groups of parental lines consistently displaying heterosis in hybrids) in different plant species. For these purposes, the current Special Issue welcomes experimental work, reviews, commentaires on phenotypic, genetic, and molecular analysis of hybrid vigor in plants, with a particular focus on non-model species, as a means to offer a wide frame for discussing the recent progress in the field.